Objective lens, and method of manufacturing the same and optical pickup apparatus

ABSTRACT

An objective lens formed of a synthetic resin material having characteristics of substantially not reducing transmittance of the objective lens when a first laser beam having a first wavelength passes through the objective lens, and reducing the transmittance according to an elapsed time to apply a second laser beam when the second laser beam having a second wavelength shorter than the first wavelength and longer than a wavelength of ultraviolet light passes through the objective lens, the objective lens being irradiated with the ultraviolet light in advance for a predetermined time such that a change in the transmittance for the second laser beam is set within a predetermined range.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese Patent Application Nos. 2007-189276 and 2008-181569, filed Jul. 20, 2007 and Jul. 11, 2008, respectively, of which full contents are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens used in an optical pickup apparatus performing an operation for reading signals recorded in an optical disc and an operation for recording signals into an optical disc, an objective lens manufacturing method, and an optical pickup apparatus.

2. Description of the Related Art

An optical disc apparatus is widely used which applies a laser beam emitted from an optical pickup apparatus to a signal recording layer of an optical disc to perform a signal reading operation and a signal recording operation.

While an optical disc apparatus using an optical disc called CD or DVD is generally used, there is recently developed an optical disc apparatus using an optical disc with improved record density, i.e., the optical disc of the Blu-ray standard or HD DVD standard (High Definition Digital Versatile Disc).

Infrared light with a wavelength of 780 nm is used as the laser beam for the operation of reading signals recorded in a CD-standard optical disc, and red light with a wavelength of 650 nm is used as the laser beam for the operation of reading signals recorded in a DVD-standard optical disc.

A protective layer provided on a top surface of the signal recording layer of the CD-standard optical disc has a thickness of 1.2 mm, and a numerical aperture of an objective lens used for the signal reading operation from the signal recording layer is set to a specified value of 0.45. The protective layer provided on the top surface of the signal recording layer of the DVD-standard optical disc has a thickness of 0.6 mm, and the numerical aperture of the objective lens used for the signal reading operation from the signal recording layer is set to the specified value of 0.6.

There is used the laser beam with a shorter wavelength, for example, blue-violet light with a wavelength of 405 nm as the laser beam for the operation of reading signals recorded in the Blu-ray and HD-DVD-standard optical discs, as compared to the CD-standard and DVD-standard optical discs.

The protective layer provided on the top surface of the signal recording layer of the Blu-ray standard optical disc has a thickness of 0.1 mm, and the numerical aperture of the objective lens used for the signal reading operation from the signal recording layer is set to the specified value of 0.85.

It is required to reduce a diameter of a laser spot generated by focusing a laser beam for reproduction of signals recorded in the signal recording layer of the Blu-ray standard optical disc and for recording signals in the signal recording layer. The objective lens used for obtaining a desired laser spot shape has not only an increased numerical aperture (NA) but also a reduced focal distance, and therefore, a curvature radius of the objective lens is reduced.

An optical pickup apparatus includes a laser diode emitting the laser beam with a wavelength corresponding to the above standards and an objective lens focusing the laser beam emitted from the laser diode onto the signal recording layer of the optical discs. Although glass is generally used as a material of the objective lens, synthetic resin has recently been often used so as to produce inexpensively (see Japanese Patent Application Laid-Open Publication No. 2005-338684).

Although a price of an optical pickup apparatus can be reduced by using the objective lens made of synthetic resin, synthetic resin has a distinctive feature that characteristics are changed corresponding to a wavelength of the laser beam to be used. When an objective lens for focusing the laser beam corresponding to the Blu-ray standard and the HD-DVD standard, i.e., a blue-violet laser beam with a wavelength of 405 nm is made of synthetic resin, which is used for the objective lens for focusing the laser beam corresponding to the DVD standard, i.e., a red laser beam with a wavelength of 650 nm, for example, synthetic resin called APEL 5014DP of Mitsui Chemicals, inc., transmittance of the objective lens is reduced by the blue-violet laser beam.

FIG. 4 is a characteristic view showing change in transmittance with time in the case of using the objective lens, as it is, which is made of the above synthetic resin called APEL 5014DP to apply the blue-violet laser beam with a wavelength of 405 nm. It is experimentally confirmed that a change in the transmittance of the objective lens with time according to the application time is greater than such specifications applicable to the objective lens as to satisfy optical performance of the optical pickup apparatus. That is, it is confirmed that the above change exceeds the predetermined range of 2%, which is set for the case of 1000-hour application of the blue-violet laser beam having a wavelength of 405 nm, so that the change goes out of the predetermined range to reduce the transmittance.

As the transmittance of the objective lens is reduced as above, intensity of the laser beam focused on the surface of the signal recording layer of the optical disc is reduced, thus reproducing and recording operations cannot normally be performed for signals recorded in the optical disc. Although synthetic resin, which is unaffected by the blue-violet laser beam, for example, synthetic resin called ZEONEX 340R of Zeon Corporation, is used as a material of the objective lens to solve such problem, this synthetic resin is expensive as compared to the material used for the objective lens for DVD, which cause a problem that the price of the optical pickup apparatus cannot be reduced.

SUMMARY OF THE INVENTION

An objective lens according to an aspect of the present invention is formed of a synthetic resin material having characteristics of substantially not reducing transmittance of the objective lens when a first laser beam having a first wavelength passes through the objective lens, and reducing the transmittance according to an elapsed time to apply a second laser beam when the second laser beam having a second wavelength shorter than the first wavelength and longer than a wavelength of ultraviolet light passes through the objective lens, the objective lens being irradiated with the ultraviolet light in advance for a predetermined time such that a change in the transmittance for the second laser beam is set within a predetermined range.

Other features of the present invention will become apparent from descriptions of this specification and of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For more thorough understanding of the present invention and advantages thereof, the following description should be read in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view showing an optical pickup apparatus including an objective lens according to one embodiment of the present invention;

FIG. 2 is a characteristic view showing a relation between transmittance of an objective lens and time according to one embodiment of the present invention;

FIG. 3 is a characteristic view for explaining a manufacturing method of an objective lens according to one embodiment of the present invention; and

FIG. 4 is a characteristic view of a relation between transmittance of an objective lens and time

DETAILED DESCRIPTION OF THE INVENTION

At least the following details will become apparent from descriptions of this specification and of the accompanying drawings.

In an embodiment according to the present invention, ultraviolet light is applied to an objective lens made of synthetic resin material having characteristics that transmittance of the objective lens is not substantially reduced for a first laser beam having a first wavelength and is reduced as the application time (irradiation time) elapses for a second laser beam having a second wavelength shorter than the first wavelength and longer than a wavelength of the ultraviolet light, and by applying the ultraviolet light to the objective lens for a predetermined time, the transmittance for the second laser beam is changed to be set to a predetermined transmittance, so that a change in the transmittance for the second laser beam is restrained. The terms that the transmittance of the objective lens for the first laser beam having the first wavelength is not substantially reduced means that the transmittance of the objective lens for the first laser beam having the first wavelength is not reduced to the extent that the reproducing and recording operations cannot normally be performed for signals recorded in the optical disc. According to an embodiment of the present invention, the objective lens can be manufactured by using an inexpensive synthetic resin material.

In an embodiment according to the present invention, the transmittance is set by adjusting the application time of ultraviolet light for the objective lens. According to an embodiment of the present invention, the transmittance can advantageously be set in accordance with characteristics of an optical pickup apparatus.

In FIG. 1, reference numeral 1 denotes a laser diode emitting a blue-violet laser beam with a wavelength of 405 nm, for example, and reference numeral 2 denotes a diffraction grating that the laser beam emitted from the laser diode 1 enters and is made up of a diffraction grating portion 2 a, which splits the laser beam into a main beam of 0th order light and sub-beams of +1st order light and −1st order light, and a half-wave plate 2 b which converts incident laser light into linear polarized light in the S direction.

Reference numeral 3 denotes a polarizing beam splitter that the laser beam having passed through the diffraction grating 2 enters and that is provided with a control film reflecting an S-polarized laser beam and allowing a laser beam polarized in the P-direction to pass therethrough. Reference numeral 4 denotes a monitoring photodetector disposed at a position where the laser beam emitted from the laser diode 1 and having passing through the polarizing beam splitter 3 is applied, and the detection output thereof is used to control output of the laser beam to be emitted from the laser diode 1.

Reference numeral 5 denotes a quarter-wave plate that is disposed at a position where the laser beam reflected by the control film 3 a of the polarizing beam splitter 3 is applied, and that converts the incident laser beam from the linear polarized light into circular polarized light. Reference numeral 6 denotes a collimating lens that the laser beam having passed through the quarter-wave plate 5 enters, that converts the incident laser beam into parallel light, and that corrects spherical aberration due to a protective layer of a Blu-ray standard optical disc D.

Reference numeral 7 denotes a reflection mirror that the laser beam having converted into parallel light by the collimating lens 6 enters and reflects the laser beam, and that return light reflected from a signal recording layer L of the optical disc D enters and reflects the return light in a direction of the polarizing beam splitter 3, which will be described later.

Reference numeral 8 denotes a sensor lens that the return light having passed through the control film 3 a included in the polarizing beam splitter 3 enters, and that has a cylindrical face, a flat face, a concave curved face or a convex curved face, etc., formed on the incident surface side and the emitting surface side. Such sensor lens 8 is provided for generating astigmatism in the return light to generate a focus error signal to be used for a focus control operation. Reference numeral 9 denotes a photodetector disposed at a position where the return light having passed through the sensor lens 8 is focused and applied, and that is made up of a four-divided sensor or the like including arrayed photodiodes. A configuration of such photodetector 9, an operation of generating the focus error signal using an astigmatic method, etc., are well known and will be omitted.

Reference numeral 10 denotes an objective lens that the laser beam reflected by the reflection mirror 7 enters, that focuses the incident laser beam on the signal recording layer L included in the optical disc D. The objective lens is manufactured by forming of synthetic resin and is designed such that a curvature radius is reduced unlike a spherical surface.

When the reproducing operation is performed for signals recorded in the optical disc, a drive current is supplied to the laser diode 1 and the laser diode 1 emits the laser beam with a wavelength of 405 nm from the laser diode 1. The laser beam emitted from the laser diode 1 is made incident on the diffraction grating 2, to be divided into 0th order light, +1st order light, and −1st order light by the diffraction grating portion 2 a making up the diffraction grating 2 and converted into the linear polarized light in the S direction by the half-wave plate 2 b. The laser beam having passed through the diffraction grating 2 is made incident on the polarizing beam splitter 3 to be reflected by the control film 3 a included in the polarizing beam splitter 3, and a portion of the incident laser beam passes through the control film 3 a to be applied to the monitoring photodetector 4.

The laser beam reflected by the control film 3 a is made incident on the collimating lens 6 through the quarter-wave plate 5 to be converted into the parallel light due to an effect of the collimating lens 6. The laser beam having converted into the parallel light by the collimating lens 6 is reflected by the reflection mirror 7 to be made incident on the objective lens 10. The laser beam made incident on the objective lens 10 is applied as a spot to the signal recording layer L of the optical disc D through the focusing operation of the objective lens 10. The laser beam emitted from the laser diode 1 is applied as a desired spot to the signal recording layer L of the optical disc D, and the numerical aperture of the objective lens 10 is set at 0.85 in this case.

Although spherical aberration is generated due to a difference in thickness of a protective layer between the signal recording layer L and the signal incident surface of the optical disc D when the above focusing operation of the laser beam is performed by the objective lens 10, an adjustment can be made by displacing the collimating lens 6 described in an embodiment in a light-axis direction, i.e., a direction of an arrow A or B so as to minimize this spherical aberration.

The operation is performed by the above operation to apply the laser beam emitted from the laser diode 1 to the signal recording layer L included in the optical disc D through the focusing operation of the objective lens 10, and when the applying operation is performed, the return light reflected from the signal recording layer L is made incident on the objective lens 10 from the side of the optical disc D. The return light made incident on the objective lens 10 enters the polarizing beam splitter 3 through the reflection mirror 7, the collimating lens 6, and the quarter-wave plate 5. Since the return light made incident on the polarizing beam splitter 3 is converted into the linear polarized light in the P direction, the return light passes through the control film 3 a included in the polarizing beam splitter 3.

The return light of the laser beam having passed through the control film 3 a is made incident on the sensor lens 8, and astigmatism is generated due to the effect of the sensor lens 8. The return light in which the astigmatism is generated due to the sensor lens 8 is applied to a sensor portion of a four-divided sensor etc., included in the photodetector 9 through the focusing operation of the sensor lens 8. As a result of applying the return light to the photodetector 9 in this way, the focus-error signal generating operation is performed, as is well known, with the use of a change in a spot shape of the main beam applied to the sensor portion included in the photodetector 9. The focus control operation can be performed by displacing the objective lens 10 toward a signal surface of the optical disc D with the use of such focus error signal.

Although not described in an embodiment according to the present invention, a configuration is configured such that the well known tracking control operation can be performed with the use of the +1st order light and −1st order light, i.e., the sub-beams generated by the diffraction grating 2, and thus a reading operation is performed for signals recorded in the optical disc D by performing such control operation.

Although the reading operation for the signals recorded in the optical disc D is performed as above, since a portion of the laser beam is applied to the monitoring photodetector 4 when the reading operation is performed, the monitor signal obtained from the monitoring photodetector 4 can be utilized for controlling a value of the drive current supplied to the laser diode 1.

Since the output of the laser beam can be controlled by controlling the value of the drive current supplied to the laser diode 1, an adjustment operation can be performed for the laser output required not only for the reading operation of signals recorded on the optical disc D but also a recording operation of signals into the optical disc D.

While the signal reproducing operation, etc., are performed in the optical pickup apparatus having a configuration shown in FIG. 1 as described above, a manufacturing method of the objective lens 10 will hereinafter be described, which is a gist of an embodiment of the present invention.

The objective lens 10 according to an embodiment according to the present invention is manufactured by being formed of the synthetic resin material, for example, cyclic olefin polymer resin such as the above APEL 5014DP, and such material is the same as a material used for the objective lens for performing the focusing operation of the first wavelength laser beam such as the red laser beam with a wavelength of 650 nm corresponding to the DVD-standard optical discs, for example.

An operation of applying ultraviolet light with a wavelength of 360 nm emitted from an ultraviolet light irradiation device such as a mercury xenon lamp is performed for the entire objective lens formed of such synthetic resin material. The time of application of ultraviolet light for such objective lens is, for example, about 50 minutes, and it is confirmed that such applying operation reduces the initial transmittance of the objective lens to about 95.7% for the blue-violet laser beam, i.e., the laser beam with a wavelength of 405 nm.

FIG. 2 shows a relationship between the application time and the transmittance when the blue-violet laser beam, i.e., the laser beam with a wavelength of 405 nm is applied to the objective lens having the transmittance reduced to be set at about 95.7% with the above method, i.e., by applying ultraviolet light with a wavelength of 360 nm emitted from a mercury xenon lamp. As seen from such a characteristic view, the transmittance is stable at 95.3% after about 40 hours has elapsed, and it is experimentally confirmed that a change in the transmittance of the objective lens applied with the blue-violet laser beam having a wavelength of 405 nm for 1000 hours can be restrained within a range of about 1%. That is, there can be considerably reduced a rate of the change in the transmittance of the objective lens with time for the blue-violet laser beam after the ultraviolet light is applied from the ultraviolet light irradiation device.

As described above, by irradiating the objective lens formed of the cyclic olefin polymer resin with the ultraviolet light from the ultraviolet light irradiation device, the change in the transmittance of the objective lens with time can be set within 2% for the case of 1000-hour application of the blue-violet laser beam having a wavelength of 405 nm, i.e., within a range specified as such specifications as to satisfy optical performance. Therefore, by applying the ultraviolet light to the objective lens formed of the same material as that used for the objective lens for performing the focusing operation of the red laser beam with a wavelength of 650 nm corresponding to the DVD-standard optical discs, the objective lens can be used without trouble as an objective lens focusing, onto the signal recording layer L of the optical disc D, the blue-violet laser beam, i.e., the laser beam with a wavelength of 405 nm used for performing the reproducing and recording operations for signals recorded in the Blu-ray standard optical disc D.

FIG. 3 shows a relationship between the application time and the transmittance when the blue-violet laser beam with a wavelength of 405 nm is applied to the objective lens for a different application time of the ultraviolet light emitted from a mercury xenon lamp. In FIG. 3, a transmittance of 100% indicates a transmittance of the blue-violet laser beam with a wavelength of 405 nm when no ultraviolet light is applied to the objective lens formed of the synthetic resin material same as the material used for the objective lens for performing the focusing operation of the red laser beam with a wavelength of 650 nm.

In FIG. 3, a characteristic indicated by a solid line represents a change in the transmittance of the objective lens after applying the ultraviolet light with a wavelength of 360 nm emitted from a mercury xenon lamp for about 50 minutes, as described above. It is known from this characteristic that the initial transmittance of the objective lens is reduced to be set at about 95.7% by the application of the ultraviolet light and that the transmittance of the objective lens is reduced to be stabilized at 95.3% after applying the blue-violet laser beam for 40 hours.

In FIG. 3, a characteristic indicated by a broken line represents a change in the transmittance of the objective lens after applying the ultraviolet light with a wavelength of 360 nm emitted from a mercury xenon lamp for about 7 minutes. It is known from this characteristic that the initial transmittance of the objective lens is reduced to be set at about 95% by the application of the ultraviolet light and that the transmittance of the objective lens is increased up to 97% after a lapse of 15 hours of the application time of the blue-violet laser beam. That is, since the transmittance of the objective lens applied with the ultraviolet light for about 7 minutes is changed by 2% or more in accordance with the application time of the blue-violet laser beam, it may become a problem to use such objective lens as the objective lens of the optical pickup apparatus.

The transmittance of the objective lens for the blue-violet laser beam is changed corresponding to the application condition of the ultraviolet light as described above, and an initial setting value and variation characteristics of the transmittance can be set to various values by adjusting the application time of the ultraviolet light. That is, in an embodiment described above, the application time of the ultraviolet light is set at about 50 minutes, and the initial transmittance of the objective lens for the laser beam with a wavelength of 405 nm is set at about 95.7%. In an embodiment described above, a transmittance setting operation is performed by applying to the objective lens the ultraviolet light emitted from a mercury xenon lamp, and such mercury xenon lamp has a characteristic that spectral intensity and width are great and a spectrum including an emission line group is broad in the ultraviolet region. As a result, since it is difficult to achieve uniform intensity distribution, etc., of ultraviolet light when ultraviolet light is applied to the objective lens, the applying operation is required to be performed with accuracy in order to obtain a highly accurate objective lens.

The initial transmittance of the objective lens can be set at a desired value by applying the ultraviolet light generated from the ultraviolet light irradiation device as above to the objective lens formed of synthetic resin for a predetermined time, and the initial transmittance can be set based on the intensity and the application time of the ultraviolet light. That is, since the application time is considerably changed in accordance with a radiant heat quantity of the ultraviolet light applied to the objective lens, the intensity and the application time of the ultraviolet light is appropriately set so as to reduce considerably the rate of the change in the transmittance of the objective lens for the blue-violet laser beam for a predetermined time, and thus, the transmittance of the objective lens can be set at a desired transmittance.

While the transmission characteristics of the objective lens formed of synthetic resin for the blue-violet laser beam are changed by applying the ultraviolet light generated from the ultraviolet irradiation device, since there is frequently employed a method by which the objective lens in the optical pickup apparatus is adhesively fixed to a member called a lens holder with an ultraviolet curing adhesive, if the ultraviolet light applied when performing such an adhesive fixing operation is applied to the objective lens, the adhesive fixing operation and a transmittance changing operation can advantageously be performed at a time.

When comparing refractive indexes between APEL 5014DP used as a material of the objective lens 10 of an embodiment according to the present invention and ZEONEX 340R that is the synthetic resin material suitable for the blue-violet laser beam, the refractive indexes of APEL 5014DP and ZEONEX 340R are 1.5434 and 1.509, respectively, for the d-line (laser beam with a wavelength of 587.6 nm, which is an emission line of a Na-lamp light source) at 25 degrees Celsius, and the refractive indexes thereof for the blue-violet laser beam with a wavelength of 405 nm are 1.5575 and 1.5215, respectively.

In this way, it can be seen that APEL 5014DP used as the synthetic resin material for the objective lens for DVD using the red laser beam, i.e., the synthetic resin material used as the material of the objective lens of an embodiment according to the present invention is greater in refractive index than the synthetic resin material used for the objective lens for the blue-violet laser beam. Therefore, since the objective lens manufactured according to an embodiment of the present invention has a greater refractive index, a curvature radius of the objective lens can be increased, and thus an embodiment according to the present invention achieves a highly advantageous effect when manufacturing an objective lens of an optical pickup apparatus requiring a short-focus objective lens.

While the transmittance is changed by applying the ultraviolet light to the objective lens made of synthetic resin in an embodiment according to the present invention, the collimating lens 6 and the sensor lens 8 making up an optical system of the optical pickup apparatus shown in FIG. 1 can be formed of synthetic resin to be configured such that the transmittances can be changed by applying the ultraviolet light to the collimating lens 6 and the sensor lens 8.

The above embodiments of the present invention are simply for facilitating the understanding of the present invention and are not in any way to be construed as limiting the present invention. The present invention may variously be changed or altered without departing from its spirit and encompass equivalents thereof. 

1. An objective lens formed of a synthetic resin material having characteristics of substantially not reducing transmittance of the objective lens when a first laser beam having a first wavelength passes through the objective lens, and reducing the transmittance according to an elapsed time to apply a second laser beam when the second laser beam having a second wavelength shorter than the first wavelength and longer than a wavelength of ultraviolet light passes through the objective lens, the objective lens being irradiated with the ultraviolet light in advance for a predetermined time such that a change in the transmittance for the second laser beam is set within a predetermined range.
 2. The objective lens of claim 1, wherein the second laser beam includes a blue-violet laser beam.
 3. The objective lens of claim 2, wherein the first laser beam includes a red laser beam.
 4. The objective lens of claim 1, wherein the transmittance for the second laser beam is set according to the time to be irradiated with the ultraviolet light.
 5. The objective lens of claim 1, wherein the synthetic resin material includes a material with a refractive index for d-line equal to 1.52 or greater at an environmental temperature of 25 degrees Celsius.
 6. An objective lens formed of a synthetic resin material having characteristics of reducing transmittance of the objective lens according to an elapsed time to apply the blue-violet laser beam when blue-violet laser beam passes through the objective lens, the objective lens being irradiated in advance with ultraviolet light emitted from an ultraviolet irradiation device for a predetermined time such that a change in the transmittance for the blue-violet laser beam is set within a predetermined range.
 7. An objective lens formed of a synthetic resin material with a refractive index for d-line equal to 1.52 or greater at an environmental temperature of 25 degrees Celsius, the objective lens being irradiated with ultraviolet light generated from an ultraviolet light irradiation device in advance for a predetermined time such that a change in transmittance for blue-violet laser beam is set within a predetermined range.
 8. A method of manufacturing an objective lens, comprising: forming an objective lens out of a synthetic resin material having characteristics of substantially not reducing transmittance of the objective lens when a first laser beam having a first wavelength passes through the objective lens and reducing the transmittance according to an elapsed time to apply a second laser beam when the second laser beam having a second wavelength shorter than the first wavelength and longer than a wavelength of ultraviolet light passes through the objective lens; and irradiating the objective lens with the ultraviolet light for a predetermined time such that a change in the transmittance of the second laser beam is set within a predetermined range.
 9. The objective lens manufacturing method of claim 8, wherein the second laser beam includes a blue-violet laser beam.
 10. The objective lens manufacturing method of claim 9, wherein the first laser beam includes a red laser beam.
 11. The objective lens manufacturing method of claim 8, wherein the transmittance for the second laser beam is set according to the time to be irradiated with the ultraviolet light.
 12. A method of manufacturing an objective lens, comprising: forming the objective lens out of a synthetic resin material having characteristics of reducing transmittance of the objective lens according to an elapsed time to apply the blue-violet laser beam when blue-violet laser beam passes through the objective lens, and irradiating the objective lens with ultraviolet light emitted from an ultraviolet irradiation device for a predetermined time such that a change in the transmittance for the blue-violet laser beam is set within a predetermined range.
 13. A method of manufacturing an objective lens, comprising: forming the objective lens with a synthetic resin material with a refractive index for d-line equal to 1.52 or greater at an environmental temperature of 25 degrees Celsius; and irradiating the objective lens with ultraviolet light generated from an ultraviolet light irradiation device for a predetermined time such that a change in transmittance for blue-violet laser beam is set within a predetermined range.
 14. A method of manufacturing an objective lens, comprising: forming an objective lens out of a synthetic resin material having characteristics of substantially not reducing transmittance of the objective lens when a first laser beam having a first wavelength passes through the objective lens and reducing the transmittance according to an elapsed time to apply a second laser beam when the second laser beam having a second wavelength shorter than the first wavelength and longer than a wavelength of ultraviolet light passes through the objective lens; and irradiating the objective lens and an ultraviolet curing adhesive with the ultraviolet light for a predetermined time at a time, when the objective lens is adhesively fixed to a lens holder with the ultraviolet curing adhesive.
 15. An optical pickup apparatus, comprising: an objective lens formed of a synthetic resin material having characteristics of substantially not reducing transmittance of the objective lens when a first laser beam having a first wavelength passes through the objective lens and reducing the transmittance according to an elapsed time to apply a second laser beam when the second laser beam having a second wavelength shorter than the first wavelength and longer than a wavelength of ultraviolet light passes through the objective lens, the objective lens being irradiated with the ultraviolet light in advance for a predetermined time such that a change in the transmittance for the second laser beam is set within a predetermined range; and a lens holder having the objective lens disposed thereon.
 16. An optical pickup apparatus, comprising: an objective lens formed of a synthetic resin material having characteristics of reducing transmittance of the objective lens according to an elapsed time to apply the blue-violet laser beam when blue-violet laser beam passes through the objective lens, and irradiated in advance with ultraviolet light emitted from an ultraviolet irradiation device for a predetermined time such that a change in the transmittance for the blue-violet laser beam is set within a predetermined range, and a lens holder having the objective lens disposed thereon.
 17. An optical pickup apparatus, comprising: an objective lens formed of a synthetic resin material with a refractive index for d-line equal to 1.52 or greater at an environmental temperature of 25 degrees Celsius, and irradiated with ultraviolet light generated from an ultraviolet light irradiation device in advance for a predetermined time such that a change in transmittance for blue-violet laser beam is set within a predetermined range; and a lens holder having the objective lens disposed thereon.
 18. An optical pickup apparatus, comprising: an objective lens formed of a synthetic resin material having characteristics of not reducing transmittance of the objective lens when a first laser beam having a first wavelength passes through the objective lens and reducing the transmittance according to an elapsed time to apply a second laser beam when the second laser beam having a second wavelength shorter than the first wavelength and longer than a wavelength of ultraviolet light passes through the objective lens; and a lens holder adhesively fixed to the objective lens with the ultraviolet curing adhesive, the objective lens and the ultraviolet curing adhesive being irradiated with the ultraviolet light for a predetermine time at a time, when the objective lens is adhesively fixed to the lens holder with the ultraviolet curing adhesive. 